1. Field of the Invention
The invention relates to electrical inductive apparatus, including new and improved magnetic core structures, and new and improved methods of constructing electrical inductive apparatus.
2. Description of the Prior Art
Stacked magnetic cores for large electrical power transformers of the core-form type conventionally use the butt-lap type of joint disclosed in U.S. Pat. No. 2,300,964. In the butt-lap joint the ends of the leg and yoke laminations are mitered and butted together to form diagonal joints between the laminations, in each layer of laminations. In principle, the joints in alternate layers are aligned, and offset from aligned joints in the intervening layers. In practice, to reduce handling, the joints in three adjacent layers of laminations are usually aligned, and the joints in the next three adjacent layers are aligned, but offset from the joints of the adjacent group of three laminations.
While the butt-lap construction can form a good magnetic circuit, it has disadvantages. One is the great care with which laminations must be stacked in order to optimize magnetic performance. Another disadvantage is the amount of power loss at the joints (true watts loss or T.W.), which increases the excitation current required (apparent watts loss or A.W.), and increases the sound level.
A step-lap joint, such as disclosed in U.S. Pat. No. 3,153,215, reduces core losses, it reduces the excitation current requirements, and it reduces the sound level, compared with a similarly rated transformer constructed with a butt-lap joint. In a step-lap joint, the joints created by the butting laminations of each layer are successively offset in succeeding layers in the same direction to create at least three "steps", and preferably at least six or seven, before the step pattern is repeated.
In the step-lap joint, induction (flux lines per unit area) is only a fraction of that in the laminations leading to the joint, as the flux spreads out where it crosses the lap portion of the joint. A butt-lap joint, in contrast, has about twice as much induction at the joint as in the laminations leading to the joint, as the flux lines crowd where the air gaps are bridged. In the butt-lap joint, eddy currents representing lost energy are generated by flux, at high induction, crossing several laminations. Eddy currents generated by flux of such orientation are restricted only by the relatively large area of the plane of the steel sheet, rather than by the small sheet thickness.
Thus, reluctance of the step-lap joint is much lower than that of the butt-lap joint, the core losses are lower, and the no-load excitation current required for a core with step-lap joints is considerably less than that for a butt-lap core. The result is achievement of a given performance level with greater efficiency and smaller unit size. Sound level is less because the much lower induction at the joints results in less "motor-action" vibration at the joints.
While the step-lap core has all of the above-mentioned advantages in true watts loss (TW), apparent watts loss (AW), and sound level, the step-lap joint has primarily been applied to the lower power ratings of core-form construction where the winding leg is rectangular in cross sectional configuration, and the windings are substantially rectangular in cross sectional configuration. The larger KVA core-form power transformers conventionally utilize round coils and cruciform core-leg cross sectional configurations. The butt-lap joint has been retained in this type of construction because the manufacturing cost of constructing the step-lap joint in a cruciform core offset the advantages to be gained.
Thus, it would be desirable to provide a new and improved step-lap core, and new and improved methods of constructing electrical inductive apparatus which utilize a step-lap core, to facilitate the manufacture thereof such that the advantages of the step-lap core are not offset by higher assembly costs.